Swanson had his contract purchased from Triple-A Tacoma on Tuesday.
Swanson, one of three players acquired from the Yankees in exchange for James Paxton, was added to the Mariners' 40-man roster in advance of the Rule 5 draft. The 25-year-old posted a combined 2.66 ERA, 1.03 WHIP and 139:29 K:BB across 121.2 innings with Double-A Trenton and Triple-A Scranton/Wilkes-Barre in 2018. With his move to Seattle, Swanson will be a candidate to start for the Mariners at some point in 2019.